In recent years, a hierarchical storage apparatus has been commercialized in which a disk device and a tape library device are combined, or a quickly accessible disk device and a slow disk device are combined.
FIG. 6 illustrates a configuration of a hierarchical storage apparatus. The hierarchical storage apparatus 601 has a function of automatically retaining the data in an optimum save place (a disk device 603 or a library device 604) by a control device 602, in accordance with a level of a freshness or reference frequency of data (content). The hierarchical storage apparatus 601 retains data that are high in reference frequency or comparatively new in the quickly accessible disk device 603, and migrates old data to the library device 604, which has data access that is slow. Then, the hierarchical storage apparatus 601 has a function of referring to the data by carrying out a retrieval thereof from the library device 604 to the disk device 603.
FIG. 7 illustrates an amount of data in the disk device. Content is deleted from the disk device 603 until it occupies several percent of a whole capacity of a disk (for example, a lower threshold limit or more, and an upper threshold limit or less), after being migrated onto the library device 603. Then, a use area on the disk is secured, realizing an increase in content retention efficiency.
In a content management in the hierarchical storage apparatus 601, there is a hierarchical storage apparatus having a function whereby a life span (a retention period) of content is optionally determined, and the content is retained without being deleted during the retention period. Also, there is also a hierarchical storage apparatus 601 that has a function capable of postponing an expiration of the retention period.
Also, in the content management in the hierarchical storage apparatus 601, for a plurality of items of content, shared information of data thereof is retained as a single item of data, and only information portions differing between the items of content are managed as individual items of content information. There is also a hierarchical storage apparatus that has a function of using a shared information portion and an individual information portion by retrieving them separately at a retrieval time (a single instance function).
A data storage apparatus which, when causing a file to migrate, saves a copy not only to a removable medium, but simultaneously to an inexpensive second cache is described in JP-A-2004-157859.
However, as a content migration method in the hierarchical storage apparatus is executed based only on information on the data's age, it is impossible to efficiently carry out a migration. Therefore, is a problem in that a disk area is depleted at an early stage.
Also, in a case of wanting to collectively change items of content having identical retention periods, it is necessary to retrieve content from a tape library onto a disk, but in the event that a migration of content is executed based only on an age of content data, there is also a possibility of saving the content to a differing tape medium. For this reason, it is necessary to make an access to a plurality of tape media. Therefore, has also been a problem of requiring time to retrieve content on the tape library.
Also, there is also a problem in that, since a time taken to retrieve large size content from the tape library is long, it is impossible to achieve a satisfying performance in response to a retrieval request.